The use of transparent plastic materials in place of glass panels is becoming more widespread. For instance, transparent glazing made of synthetic organic polymers is now employed more frequently in transportation, such as trains, buses and the like, in optical equipment, and in construction materials. In comparison with glass, transparent plastics are shatter-resistant and lighter in weight.
While enjoying the foregoing advantages, transparent plastics are nevertheless susceptible to scratching and marring on the surface, which impairs visibility and detracts from the physical appearance. Moreover, transparent plastics tend to undergo discoloration upon prolonged exposure to ultraviolet light, e.g., sunlight.
Attempts have been made to improve the abrasion resistance of transparent plastics. It has been proposed, for instance, that mar- or scratch-resistant coatings for such plastics can be prepared from mixtures comprising silica and hydrolyzable silanes in a suitable medium such as alcohol and water. Such protective coatings are described in Misch et al., U.S. Pat. Nos. 3,708,225, Clark, 3,986,997 and 3,976,497 and Ubersax, 4,177,315. Other scratch resistant protective coatings are described in copending applications Ser. No. 964,910 and Ser. No. 964,911, both filed Nov. 30, 1978, assigned to the same assignee as herein. Typically, these are applied to the surface of the transparent plastic and heat cured in situ.
Attempts have also been made to reduce the tendency of the coatings on transparent plastics toward deterioration on exposure to sunlight and the like by incorporating in the coating compositions materials which absorb ultraviolet light rays. A disadvantage of many such ultraviolet light absorbing materials is that they often escape from the compositions, e.g., by volatilization, usually during the heat curing cycle. Efforts to overcome this defect by using ultraviolet light absorbing materials having higher molecular weights, e.g., American Cyanamid's Cyasorb UV-531, have not been entirely successful.
Proskow, U.S. Pat. No. 4,051,161, discloses an alternative approach and this is to use a silane-fluorohydroxy copolymer coating with a silanol-reactive functional derivative of an aromatic ultraviolet light-absorbing compound. As the agent to contribute the silanol-reactive functional group it is proposed to use a complex epoxy-silane compound. The Proskow coating is not the preferred, less complex silica-hydrolyzable silane coating of the earlier mentioned citations. Other commonly assigned, concurrently filed applications also deal with functionalized uv screens. See U.S. Pat. No. 4,278,804 (Ashby et al.); copending U.S. application Ser. No. 154,623 (Ching), filed May 30, 1980; copending U.S. application Ser. No. 154,625 (Ching), filed May 30, 1980; and U.S. application Ser. No. 154,626 (Ching), now allowed, filed May 30, 1980.
The disclosures of the foregoing patents and applications are incorporated herein by reference.
It has now been discovered that reactive functional derivatives of aromatic ultraviolet light-absorbing agents with superior properties in all important respects can be obtained by using alkoxysilylalkylor alkanoyloxysilylalkylcarbamyl functional groups, and that these are useful in the less complex systems, i.e., not the copolymer coating systems called for in U.S. Pat. No. 4,051,161.
With the new compounds of this invention scratch-resistant coatings for transparent plastics can be made more resistant to discoloration upon exposure to ultraviolet light. Because such modified ultraviolet light absorbers are adapted to coreact with the polysiloxane of the scratch-resistant coating composition, there is a much reduced tendency of such materials to escape or oxidize during thermal processing. This provides substantial economy of use in comparison with the prior art.